The diginal natural gamma-ray spectra logging tool can be used in open hole or cased hole. By separate the gamma ray spectrum into it Uranium (U), Thorium (Th), Potassium (K) components, the log analysts can identify the lithology of formations, locating zones, calculation the shale content, evaluate the sediment conditions, measusing bed thickness, and makeing qualitative estimates of formation permeability. The spectralog also can provide standard curve for depth correction.